1. Field of the Invention
The present invention relates to a ceramic heater used in various applications of heating and ignition, particularly to a ceramic heater having excellent durability and a method for manufacturing the same.
2. Description of the Related Art
Ceramic heaters are widely used in various applications such as heating of various sensors, glow plug system, heating of semiconductor and ignition of kerosene burning fan heater.
There are various ceramic heaters according to applications.
For the heating element of air-fuel ratio sensor of automobile, carburetor heater for automobile, soldering iron heater and the like, for example, such a ceramic heater is commonly used that comprises a heat generating resistor made of a metal having high melting point such as W, Re or Mo incorporated in a ceramic member that is constituted from a main component of alumina as described in, for example, Patent Documents 1 through 3.
Ignition heaters of various combustion apparatuses such as kerosene burning fan heater and gas burning boilers, as well as heaters for measuring instruments are required to have durability at high temperatures. These heaters are also often used with high voltages beyond 100 V applied thereto. Accordingly, ceramic heaters made of silicon nitride ceramics as the base material and using WC that has a high melting point and a thermal expansion coefficient proximate to that of the base material is commonly used for the heat generating resistor. The heat generating resistor may also contain BN or silicon nitride powder added thereto for the purpose of making the thermal expansion coefficient thereof proximate to that of the base material of the ceramic heater (refer to Patent Document 4). Thermal expansion coefficient of the base material may also be made proximate to that of the heat generating resistor by adding an electrically conductive ceramic material such as MoSi2, WC or the like to the base material (refer to Patent Document 5).
A ceramic heater made by using silicon nitride ceramics as the base material is also used in an onboard heater of automobile. The onboard heater of automobile is used as a heat source that enables it to quickly start an automobile engine in cold climate or an auxiliary heat source that assists heating automobile passenger room, and uses a liquid fuel. In an electric vehicle, limitation on the capacity of the battery requires it to decrease the consumption of electricity, and it is envisioned to use an onboard heater that uses the liquid fuel as the heat source of the passenger room heater. The ceramic heater used in the onboard heater of automobile is required to have a long service life, and to be integrated with a thermistor that senses the combustion temperature. In order to integrate the ceramic heater and the thermistor, the ceramic heater must have high durability and the change in resistance must be small over a long period of use.
Ceramic heaters may be formed in various shapes including cylinder and flat plate. A ceramic heater having cylindrical shape is manufactured by such a method as described in Japanese Unexamined Patent Publication (Kokai) No. 2001-126852. A ceramic rod and a ceramic sheet are prepared, and a paste of metal that has a high melting point consisting of a metal of one kind selected from among W, Re and Mo is printed onto one side of the ceramic sheet so as to form a heat generating resistor and a lead-out section. Then the ceramic sheet is wound around the ceramic rod with the side whereon the heat generating resistor and the lead-out section facing inside. While the operation of winding the ceramic sheet around the ceramic rod is carried out manually, the winding is tightened by means of a roller apparatus in order to achieve firm contact between the ceramic sheet and the ceramic rod (Patent Documents 6 and 7). Then the assembly is fired so as to consolidate into a nomolithic body. A lead-out section formed on the ceramic sheet is connected to an electrode pad via through hole that is formed in the ceramic sheet. The through hole is filled with an electrically conductive paste as required.    Patent Document 1: Japanese Unexamined Patent Publication (Kokai) No. 2002-146465    Patent Document 2: Japanese Unexamined Patent Publication (Kokai) No. 2001-126852    Patent Document 3: Japanese Unexamined Patent Publication (Kokai) No. 2001-319757    Patent Document 4: Japanese Patent Unexamined Publication No. 7-135067    Patent Document 5: Japanese Unexamined Patent Publication (Kokai) No. 2001-153360    Patent Document 6: Japanese Unexamined Patent Publication (Kokai) No. 2000-113964    Patent Document 7: Japanese Unexamined Patent Publication (Kokai) No. 2000-113965